Self-healing materials have received considerable attention due to their great potential to diminish degradation and reduce the maintenance cost. Since the first generation self-healing material based on the ring opening metathesis polymerization (ROMP) of encapsulated dicyclopentadiene (DCPD) in the presence of Grubbs' catalyst particles (S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794-797) microencapsulation has been one of the most efficient and widely used approaches in self-healing materials development. Poly(urea-formaldehyde) (PUF) microcapsules containing DCPD as healing agent were prepared through an in situ polymerization in oil-in-water emulsion (E. Brown, M. Kessler, N. Sottos and S. White, J. Microencapsulation, 2003, 20, 719-730; M. R. Kessler, N. R. Sottos and S. R. White, Composites: Part A, 2003, 34, 743-753) and the capsules size was further reduced to nanometre scale with the assistance of a sonication technique (B. J. Blaiszik, N. R. Sottos and S. R. White, Compos. Sci. Technol., 2008, 68, 978-986). Linseed oil (C. Suryanarayana, K. C. Rao and D. Kumar, Prog. Org. Coat., 2008, 63, 72-78), amines (D. A. McIlroy, B. J. Blaiszik, M. M. Caruso, S. R. White, J. S. Moore and N. R. Sottos, Macromolecules, 2010, 43, 1855-1859) and epoxy resins (L. Yuan, G. Liang, J. Xie, L. Li and J. Guo, Polymer, 2006, 47, 5338-5349) were also microencapsulated for self-healing applications. To avoid the contamination of catalyst by the host matrix, a dual capsule system was reported (S. Cho, H. Andersson, S. White, N. Sottos and P. Braun, Adv. Mater., 2006, 18, 997-1000; S. H. Cho, S. R. White and P. V. Braun, Adv. Mater., 2009, 21, 645-649) and this approach has shown good self-healing and corrosion protection features. Most of the capsules applied for self-healing purpose so far were made from PUF, polyurethane (PU) and polyurea. As an alternative, a double-walled polyurethane-poly(urea formaldehyde) (PU-PUF) microcapsule was recently developed through the combination of interfacial polymerization of PU and in situ polymerization of PUF in a single batch reaction (M. M. Caruso, B. J. Blaiszik, H. Jin, S. R. Schelkopf, D. S. Stradley, N. R. Sottos, S. R. White and J. S. Moore, ACS Appl. Mater. Interfaces, 2010, 2, 1195-1199.) Other approaches such as hollow glass fiber embedment, microvascular system, and electrospun hollow fibers have also been extensively investigated for self-healing materials development, and more recently there was reported an oxetane-substituted chitosan precursor incorporated PU showing good scratch closure performance within half an hour under sunlight (B. Ghosh and M. Urban, Science, 2009, 323, 1458-1460).
Since isocyanates are reactive with moisture and can thus be used as a potential healing agent to develop one-part, catalyst-free self-healing materials that are exposed to moist or aqueous environments, their suitability as self-healing materials has been rather extensively. Early research on encapsulation of isocyanate has been mainly restricted to its blocked form or solid state (I. W. Cheong and J. H. Kim, Chem. Commun., 2004, 2484-2485; H. Yang, S. Mendon and J. Rawlins, eXPRESS Polym. Lett., 2008, 2, 349-356) while Yang et, al. for the first time reported in Macromolecules, 2008, 41, 9650-9655, the microencapsulation of liquid isocyanate monomer. Less reactive isophorone diisocyanate (IPDI) was encapsulated by polyurethane microcapsules based on the polymerization of toluene diisocyanate (TDI) prepolymer that was cautiously in-house synthesized.
Silanes are a further class of potential seal-healing materials. However, to date, the research on organic silane for self-healing materials remains largely unexplored, and only a few publications have appeared. Braun and co-workers reported the polydimethylsiloxane (PDMS)-based silane microcapsules applied for self-healing coatings (S. H. Cho, S. R. White, P. V. Braun, Advanced Materials 2009, 21, 645 and S. Cho, H. Andersson, S. White, N. Sottos, P. Braun, Advanced Materials 2006, 18, 997). A mixture of hydroxyl end-functionalized PDMS (HOPDMS) and polydiethoxysiloxane was directly phase-separated, or encapsulated and then dispersed in epoxy matrix, and the yielding coating exhibited good self-healing ability. However, organo-tin catalyst was necessary for such self-healing system. Another example is the microencapsulation of a self-synthesized silyl ester for self-healing coatings reported by S. J. García, H. R. Fischer, P. A. White, J. Mardel, Y. González-García, J. M. C. Mol, A. E. Hughes, Progress in Organic Coatings, 70, 142. Octyldimethylsilyloleate (silyl ester) was synthesized and encapsulated into poly(urea-formaldehyde) (PUP) microcapsules, which were then incorporated into epoxy coating to produce a self-healing coating the self-healing properties of which were demonstrated.
However, there is still a need to provide further self-healing materials and microcapsules that are suitable for the microencapsulation of such self-healing materials.